llvm::PointerType::isValidElementType() checks that the pointee type is
not void. Use a byte pointer instead.

Bug: b/155148722
Change-Id: Ibf2fd23ce0a19a6947a6817f0b73753cfa170832
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49568Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>